It is becoming increasingly more common for wheelchairs wheels to be provided with gears in order to facilitate use thereof for different users, especially when travelling on surfaces having varying gradients. This is especially the case for wheels intended for use on manually powered wheelchairs.
Users' success in operating a manually powered wheelchair and doing so independently depends greatly on their ability to power it themselves. Whereas some individuals with manual wheelchairs have access to staff for pushing the wheelchair, it is usually desirable for users themselves to be able to propel their wheelchairs.
Manual wheelchairs are usually propelled, steered and braked by users, either by turning or gripping a ring attached to each wheel of the wheelchair. Each such gripping ring is structurally connected to the respective wheel of the wheelchair, the latter being in contact with the running surface. Users usually have to exert a relatively large force to effect many of the operations involved in using a conventional wheelchair, by acting upon the gripping rings on either side of the wheelchair.
These operations may cause users various kinds of acute and chronic arm and shoulder damage. Other problems include elbow, wrist and hand damage caused by repetitive stress on these parts of the body. Users' hands may also sustain wear-and-tear damage due to abrasion and friction between hands and gripping rings.
Various environmental factors may accelerate or even cause said damage, e.g. rough ground or uneven surfaces on which the wheelchair travels. Using a wheelchair in rain or on snow or ice or with cold or slippery hands may also cause damage. In many cases, the occurrence of such damage may be a contributory factor in otherwise healthy users being compelled to change to a powered wheelchair or needing assistance to move their wheelchair.
A number of steps have already been taken in the past to improve the performance of manual wheelchairs and make their use less burdensome for users. Such improvements include two-speed wheelchairs of the kind referred to e.g. in U.S. Pat. No. 7,832,753 to the present applicant, and which is hereby included by reference in its entirety. U.S. Pat. No. 7,832,753 relates to a gear shift pertaining to a hub of a wheel with a quick lock. The hub has a gear which is supported round a shaft with an outer end and an inner end. The inner end of the shaft is suspended via a suspension device on a wheelchair. The quick lock includes a rod which runs in a cylindrical hole which is concentric within and with the shaft and a catch which locks the wheel with the shaft to the suspension device. The gear shift includes the shaft, the rod and a control, all of which are so adapted that the rod has a neutral position relative to the shaft and wherein a movement, relative to the shaft, of the rod from the neutral position in a direction from the outer end of the shaft towards the inner end of the shaft disengages the catch so that the wheel together with the shaft can be locked to and released from the suspension device. The rod is further provided with a gear selector contact configured to contact a gear selector in the gear, and, by a movement, relative to the shaft, of the rod in an axial direction from the neutral position in a direction from the inner end of the shaft to the outer end of the shaft, acts via the gear selector contact to cause the gear selector in the gear to change gear position so that the gear ratio of the wheel relative to a drive device for the wheel is changed.
In U.S. Pat. No. 7,832,753 the choice of gear position is controlled by a gear positioner, which is situated within easy reach on the wheelchair frame. The gear positioner transmits the gear positions set by the user via a control, which may take the form of a wire or other elongate flexible means or a link system, to the rod which controls the gear selector in the planetary gear. A wheelchair is usually provided with two wheels driven by gripping rings. Both wheels are, according to U.S. Pat. No. 7,832,753, said to preferably be equipped with the gear change mechanism, in which case it is stated as advantageous that the gear positioner should act on planetary gears in both wheels in order to set the same gear position for both wheels.
The gear shift according to U.S. Pat. No. 7,832,753 thus comprises a control for operating the rod in order to impart a selected gear position. According to one embodiment, the control takes the form of a wire connected to a gear positioner disposed at a location convenient to the user. According to this embodiment a lever is disposed at the end of the control, which acts upon the rod. The lever is provided with an eccentric. In a first gear change position, the eccentric on the lever does not abut against the rod. The spring force of a return spring holds the gear stationary in the first gear change position. Switching of the gear positioner, for selection of a second gear position, will cause consequent pulling of the wire, the lever pivots, with the result that the eccentric pushes the rod in and thereby causes the rod, the shaft spigot and the gear selector to set a second gear position in the gear.
Where a gear shift according to U.S. Pat. No. 7,832,753 is used on a wheelchair, it is advantageous that both of the wheelchair's wheels be subjected to approximately simultaneous gear changing and that the same gear ratio be set for both wheels.
However, although it has been found that such a wire operated gear change system works quite well a few drawbacks have been identified. Firstly, the wheel attachment adaptors used will be sensitive to external or environmental influences, such as shocks, moisture and soiling, such as from dirt, gravel, snow, ice and slush, which may cause malfunctions. Secondly, although a requirement that the wheelchair's wheels needs to be aligned is usually are met quite well immediately after initial application of the wheel attachment adaptors, the wheel adaptors are prone to become un-aligned after a period of time. This will in turn cause the wheels of the wheelchair to become un-aligned having unsuitable camber and/or toe-in, toe-out angles, having a negative impact on the usability of the wheelchair. This as the angle of the wheels to the direction of travel, the so called or toe-in and toe-out angles affect the stability of the wheelchair in the direction of travel, and furthermore, the angle to the surface travelled upon, the so called camber angle affects the ability of the wheelchair to negotiate corners, i.e. the cornering ability thereof.